Heretofore, an electronic circuit board using an insulating material has been known. FIG. 2 is a sectional view showing a structure example of such an electronic circuit board 300. In the electronic circuit board shown in FIG. 2, an insulating film 302 is formed on a metal base 301, and also a conductive foil 303 as a wiring pattern is formed thereon. On the electronic circuit board 300, a mounted circuit 304 is formed using electronic components such as an integrated circuit (IC). The respective electronic components are electrically connected to a wiring pattern formed of the conductive foil 303. The mounted circuit 304 generates heat at the time of an operation. This heat is directly radiated from IC or the like, or transmitted to a metal base 301 through the insulating film 302 and is also radiated from the metal base 301. Therefore, in the electronic circuit board using the metal base 301, high heat radiation effect can be obtained.
In order to sufficiently exert the heat radiation effect of the metal base 301, it is desired to sufficiently increase thermal conductivity of the insulating film 302. It is also required for the insulating film 302 to have sufficient performances with respect to heat resistance and adhesion to the conductive foil 303.
The liquid crystal polyester has attracted special interest as an insulating film forming material for an electronic circuit board since it has low hygroscopicity, in addition to high thermal conductivity and excellent heat resistance. An electronic circuit board using a liquid crystal polyester film is described, for example, in JP-B-6-82893, JP-A-2004-315678 and JP-B-5-71630.
JP-B-6-82893 discloses a method in which a mixture of a liquid crystal polyester and a filler is formed into a sheet through extrusion molding to form a flat sheet, and then copper is laminated on a surface of the flat sheet to produce a printed circuit board (refer to lines 17 to 44 of left column on page 12 of JP-B-6-82893).
JP-A-2004-315678 discloses a technique in which corrosion resistance of a liquid crystal polyester solution is improved by dissolving a liquid crystal polyester containing 10 to 35 mol % of a structural unit derived from an aromatic diamine in an aprotic solvent, and a method in which anisotropy and mechanical strength of a liquid crystal polyester film for an electronic circuit board are improved by removing a solvent from a cast of this solution (refer to columns 0002 and 0039 of JP-A-2004-315678).
JP-B-5-71630 discloses a method in which thermal conductivity of a liquid crystal polyester is improved by including an insulating inorganic powder having high thermal conductivity such as an alumina powder or a magnesium powder, thereby improving heat radiation properties of a circuit board (refer to lines 24 to 39 of right column on page 10 of JP-B-5-71630).